Panel: fixed invisible column issue

FIXED: Panel columns may become invisible when settings are restored from config

After going through several version upgrades some of columns in panels
became invisible for some reason (debugging shows size = 0).
The column state is saved to config and restored each time Krusader loads,
so it was impossible to fix this without killing the krusaderrc file.

Differential Revision: https://phabricator.kde.org/D12338